Yamaha YZF-R125 Service Manual: Ecu self-diagnostic function

The ECU is equipped with a self-diagnostic function in order to ensure that the fuel injection system is operating normally. If this function detects a malfunction in the system, it immediately operates the engine under substitute characteristics and illuminates the engine trouble warning light to alert the rider that a malfunction has occurred in the system. Once a malfunction has been detected, a fault code is stored in the memory of the ECU.

  • To inform the rider that the fuel injection system is not functioning, the engine trouble warning light flashes when the start switch is being pushed to start the engine.
  • If a malfunction is detected in the system by the self-diagnostic function, the ECU provides an appropriate substitute characteristic operation, and alerts the rider of the detected malfunction by illuminating the engine trouble warning light.
  • After the engine has been stopped, the lowest fault code number is indicated by the engine trouble warning light (or displayed on the FI diagnostic tool). It remains stored in the memory of the ECU until it is deleted.

1. Engine trouble warning light

Engine trouble warning light fault code indication

Digit of 10: Cycles of 1 sec. on and 1.5 sec. off.

Digit of 1: Cycles of 0.5 sec. on and 0.5 sec. off.

Example: 42


a. Light on

b. Light off

c. 1

d. 1.5

e. 0.5

f. 3

Engine trouble warning light indication and fuel injection system operation


* The warning light flashes when any one of the conditions listed below is present and the start switch is pushed:

19: Blue/yellow ECU lead (broken or disconnected)

30: Lean angle sensor (latch up detected)

33: Faulty ignition

39: Fuel injector (open or short circuit)

41: Lean angle sensor (open or short circuit)

50: ECU internal malfunction (memory check error)

Checking the engine trouble warning light

The engine trouble warning light comes on for 3 seconds after the main switch has been set to "ON". If the warning light does not come on under these conditions, the warning light (LED) may be defective.


a. Main switch "OFF"

b. Main switch "ON"

c. Engine trouble warning light off

d. Engine trouble warning light on for 3 seconds

    Circuit diagram
    2. Crankshaft position sensor 4. Main fuse 5. Main switch 6. Radiator fan motor fuse 8. Sidestand switch 9. Battery 17.Engine stop switch 19.Intake air pressure sensor 20.Intake air temper ...

    Self-diagnostic function table
    If the ECU detects an abnormal signal from a sensor while the vehicle is being driven, the ECU illuminates the engine trouble warning light and provides the engine with alternate operating instru ...

    Other materials:

    Removing the fuel tank
    1. Extract the fuel in the fuel tank through the fuel tank filler hole with a pump. 2. Remove: Fuel hose connector cover "1" 3. Disconnect: Fuel hose "2" CAUTION: Be sure to disconnect the fuel hose by hand. Do not forcefully disconnect the hose with tools. A ...

    Installing the engine
    1. Install: Engine "1" Engine mounting bolt (rear lower side) "2" Engine mounting nut (rear lower side) "3" Engine mounting bolt (rear upper side) "4" Engine mounting nut (rear upper side) "5" Engine mounting bolt (front side) "6" Engine mounting nut (front side) "7" NOTE: Do not ...

    Checking the rear brake hose
    1. Check: Brake hoses "1" Cracks/damage/wear  Replace. 2. Check: Brake hose holder "2" Loose connection Tighten the holder bolt. Loose or open holder Fasten properly. 3. Hold the vehicle upright and apply the rear brake several times. 4. Check: Brake ho ...

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